Modem microprocessors include general purpose registers that are architecturally visible to programs. The programs use the general purpose registers to perform their necessary functions, such as arithmetical and logical calculations or movement of data between the microprocessor and memory or I/O devices. For example, within microprocessors that conform to the IA-32 Intel Architecture (also commonly known as the x86 architecture), general purpose registers include the EAX, EBX, ECX, EDX, ESI, EDI, ESP, and EBP registers, as well as the x86 floating-point register set and the multimedia-related MMX and XMM register sets.
In addition, it has become increasingly more common for microprocessors to also include registers that are accessible by programs, but that are not general purpose registers in the sense that the microprocessor restricts access to these non-general purpose registers unless the program attempting to access them has the requisite authority, or privilege, to do so. For example, within x86-compatible microprocessors exist model-specific registers (MSRs) that are control registers that may only be accessed by programs executing at the highest privilege level. The MSRs typically allow system software (i.e., privileged software) to enable, disable or configure various features of the microprocessor. In particular, the features may be specific to the microprocessor model. Examples of the features include performance, debugging, testing, monitoring, or power conservation features, among others. As an example, see Appendix B, Model-Specific Registers (MSRs), in the IA-32 Intel Architecture Software Developer's Manual, Volume 3B: System Programming Guide, Part 2, June 2006 for a list of MSRs included on various Intel microprocessor models.
Many of the features that may be controlled by MSRs are relatively benign. However, some of the features that may be controlled by MSRs can drastically affect operation of the microprocessor. Because of the potentially drastic effects, some microprocessor manufacturers have gone even one step farther than requiring a high privilege level in restricting access to particularly “dangerous” MSRs. For example, certain models of microprocessors manufactured by Advanced Micro Devices, Inc. (AMD®) require the system software to provide a password in a general purpose register in order to access a subset of the MSRs of the processor, namely those MSRs that control certain features of the microprocessor's operation that the manufacturer considers dangerous.
Password-protecting access to MSRs has benefits; however, it also adds a burden to system software that needs to access the MSRs. In particular, password-protecting access to a subset of the MSRs of a processor raises the issue of whether the manufacturer has correctly identified the correct subset of MSRs to password protect. Conventionally, the subset of password-protected MSRs is hardcoded, that is, the subset is fixed at the time the manufacturer fabricates the microprocessor. This may be problematic if the manufacturer later discovers that it has password-protected an MSR that it now wishes it had not password-protected, or has not password-protected an MSR that it now wishes it had password-protected. At this point, the manufacturer will likely have to discard the parts that have already been fabricated with the undesirable subset of password-protected MSRs, which may potentially cause the manufacturer to lose a large amount of revenue. The probability of selecting the wrong subset of MSRs to password-protect increases as the number of MSRs per model increases, and in recent times the number of MSRs has proliferated. Therefore, what is needed is a way to alter the subset of password-protected MSRs in a microprocessor after the microprocessor has been fabricated.